Multilayer optical films, i.e., films that provide desirable transmission and/or reflection properties at least partially by an arrangement of microlayers of differing refractive index, are known. It has been known to make such multilayer optical films by depositing a sequence of inorganic materials in optically thin layers (“micro layers”) on a substrate in a vacuum chamber. Inorganic. multilayer optical films are described, for example, in textbooks by H. A. Macleod, Thin-Film Optical Filters, 2nd Ed., Macmillan Publishing Co, (1986) and by A. Thelen, Design of Optical Interference Filters, McGraw-Hill, Inc. (1989).
Multilayer optical films have also been demonstrated by coextrusion of alternating polymer layers. See, e.g., U.S. Pat. Nos. 3,610,729 (Rogers), 4,446,305 (Rogers et al.), 4,540,623 (Im et al.), 5,448,404 (Schrenk et al.), and 5,882,774 (Jonza et al.). In these polymeric multilayer optical films, polymer materials are used predominantly or exclusively in the makeup of the individual layers. Such films are compatible with high volume manufacturing processes and can be made in large sheets and roll goods.
A multilayer optical film includes individual microlayers having different refractive index characteristics so that some light is reflected at interfaces between adjacent microlayers. The microlayers are sufficiently thin so that light reflected at a plurality of the interfaces undergoes constructive or destructive interference in order to give the multilayer optical film the desired reflective or transmissive properties. For multilayer optical films designed to reflect light at ultraviolet, visible, or near-infrared wavelengths, each microlayer generally has an optical thickness (a physical thickness multiplied by refractive index) of less than about 1 μm. Thicker layers are also typically included, such as skin layers at the outer surfaces of the multilayer optical film, or protective boundary layers (PBLs) disposed within the multilayer optical films, that separate coherent groupings (referred to herein as “packets”) of microlayers.
For polarizing applications, e.g., reflective polarizers, at least some of the optical layers are formed using birefringent polymers, in which the polymer's index of refraction has differing values along orthogonal Cartesian axes of the polymer. Generally, birefringent polymer microlayers have their orthogonal Cartesian axes defined by the normal to the layer plane (z-axis), with the x-axis and y-axis lying within the layer plane. Birefringent polymers can also be used in non-polarizing applications.
In some cases, the microlayers have thicknesses and refractive index values corresponding to a ¼-wave stack, i.e., arranged in optical repeat units or unit cells each having two adjacent microlayers of equal optical thickness (f-ratio=50%), such optical repeat unit being effective to reflect by constructive interference light whose wavelength λ is twice the overall optical thickness of the optical repeat unit. Other layer arrangements, such as multilayer optical films having 2-microlayer optical repeat units whose f-ratio is different from 50%, or films whose optical repeat units include more than two microlayers, are also known. These optical repeat unit designs can be configured to reduce or to increase certain higher-order reflections. See, e.g., U.S. Pat. Nos. 5,360,659 (Arends et al.) and 5,103,337 (Schrenk et al.). Thickness gradients along a thickness axis of the film (e.g., the z-axis) can be used to provide a widened reflection band, such as a reflection band that extends over the entire human visible region and into the near infrared so that as the band shifts to shorter wavelengths at oblique incidence angles the microlayer stack continues to reflect over the entire visible spectrum. Thickness gradients tailored to sharpen band edges, i.e., the wavelength transition between high reflection and high transmission, are discussed in U.S. Pat. No. 6,157,490 (Wheatley et al.).
Further details of multilayer optical films and related designs and constructions are discussed in U.S. Pat. Nos. 5,882,774 (Jonza et al.) and 6,531,230 (Weber et al.), PCT Publications WO 95/17303 (Ouderkirk et al.) and WO 99/39224 (Ouderkirk et al.), and the publication entitled “Giant Birefringent Optics in Multilayer Polymer Mirrors”, Science, Vol. 287, March 2000 (Weber et al.). The multilayer optical films and related articles can include additional layers and coatings selected for their optical, mechanical, and/or chemical properties. For example, a UV absorbing layer can be added at the incident side of the film to protect components from degradation caused by UV light. The multilayer optical films can be attached to mechanically reinforcing layers using a UV-curable acrylate adhesive or other suitable material. Such reinforcing layers may comprise polymers such as PET or polycarbonate, and may also include structured surfaces that provide optical function such as light diffusion or collimation, e.g. by the use of beads or prisms. Additional layers and coatings can also include scratch resistant layers, tear resistant layers, and stiffening agents. See e.g. U.S. Pat. No. 6,368,699 (Gilbert et al.). Methods and devices for making multilayer optical films are discussed in U.S. Pat. No. 6,783,349 (Neavin et al.).
FIG. 1 depicts one layer pair of a multilayer optical film 10. The film 10 includes a large number of alternating microlayers 12, 14, only two of which are shown for simplicity. The microlayers have different refractive index characteristics so that some light is reflected at the interfaces between adjacent microlayers. The microlayers are thin enough so that light reflected at a plurality of the interfaces undergoes constructive or destructive interference to give the film the desired reflective or transmissive properties. For optical films designed to reflect light at ultraviolet, visible, or near-infrared wavelengths, each microlayer generally has an optical thickness (i.e., a physical thickness multiplied by refractive index) of less than about 1 μm. Thicker layers are also typically included, such as skin layers at the outer surfaces of the film, or protective boundary layers disposed within the film that separate packets of microlayers.
The reflective and transmissive properties of multilayer optical film 10 are a function of the refractive indices of the respective microlayers and the thicknesses and thickness distribution of the microlayers. Each microlayer can be characterized at least in localized positions in the film by in-plane refractive indices nx, ny, and a refractive index nz associated with a thickness axis of the film. These indices represent the refractive index of the subject material for light polarized along mutually orthogonal x-, y-, and z-axes, respectively. In FIG. 1, these indices are labeled n1x, n1y, n1z for layer 12, and n2x, n2y, n2z for layer 14, their respective layer-to-layer differences being Δnx, Δny, Δnz. For ease of explanation in the present patent application, unless otherwise specified, the x-, y-, and z-axes are assumed to be local Cartesian coordinates applicable to any point of interest on a multilayer optical film, in which the microlayers extend parallel to the x-y plane, and wherein the x-axis is oriented within the plane of the film to maximize the magnitude of Δnx. Hence, the magnitude of Δny can be equal to or less than—but not greater than—the magnitude of Δnx. Furthermore, the selection of which material layer to begin with in calculating the differences Δnx, Δny, Δnz is dictated by requiring that Δnx be non-negative. In other words, the refractive index differences between two layers forming an interface are Δnj=n1j−n2j, where j=x, y, or z and where the layer designations 1,2 are chosen so that n1x≧n2x, i.e., Δnx≧0.
In practice, the refractive indices are controlled by judicious materials selection and processing conditions. Film 10 is made by co-extrusion of a large number, e.g. tens or hundreds of layers of two alternating polymers A, B, typically followed by passing the multilayer extrudate through one or more multiplication die, and then stretching or otherwise orienting the extrudate to form a final film. The resulting film is typically composed of many hundreds of individual microlayers whose thicknesses and refractive indices are tailored to provide one or more reflection bands in desired region(s) of the spectrum, such as in the visible or near infrared. To achieve high reflectivities with a reasonable number of layers, adjacent microlayers typically exhibit a difference in refractive index (Δnx) for light polarized along the x-axis of at least 0.05. If the high reflectivity is desired for two orthogonal polarizations, then the adjacent microlayers also can be made to exhibit a difference in refractive index (Δny) for light polarized along the y-axis of at least 0.05.
The '774 (Jonza et al.) patent referenced above describes, among other things, how the refractive index difference (Δnz) between adjacent microlayers for light polarized along the z-axis can be tailored to achieve desirable reflectivity properties for the p-polarization component of obliquely incident light. To maintain high reflectivity of p-polarized light at oblique angles of incidence, the z-index mismatch Δnz between microlayers can be controlled to be substantially less than the maximum in-plane refractive index difference Δnx, such that Δnz≦0.5*Δnx, or Δnz≦0.25*Δnx. A zero or near zero magnitude z-index mismatch yields interfaces between microlayers whose reflectivity for p-polarized light is constant or near constant as a function of incidence angle. Furthermore, the z-index mismatch Δnz can be controlled to have the opposite polarity compared to the in-plane index difference Δnx, i.e. Δnz<0. This condition yields interfaces whose reflectivity for p-polarized light increases with increasing angles of incidence, as is the case for s-polarized light.
The '774 (Jonza et al.) patent also discusses certain design considerations relating to multilayer optical films configured as polarizers, referred to as multilayer reflecting or reflective polarizers. In many applications, the ideal reflecting polarizer has high reflectance along one axis (the “extinction” or “block” axis, corresponding to the x-direction) and zero reflectance along the other axis (the “transmission” or “pass” axis, corresponding to the y-direction). If some reflectivity occurs along the transmission axis, the efficiency of the polarizer at off-normal angles may be reduced, and if the reflectivity is different for various wavelengths, color may be introduced into the transmitted light. Furthermore, exact matching of the two y indices and the two z indices may not be possible in some multilayer systems, and if the z-axis indices are not matched, introduction of a slight mismatch may be desired for in-plane indices n1y and n2y. In particular, by arranging the y-index mismatch to have the same sign as the z-index mismatch, a Brewster effect is produced at the interfaces of the microlayers, to minimize off-axis reflectivity, and therefore off-axis color, along the transmission axis of the multilayer reflecting polarizer.
Another design consideration discussed in '774 (Jonza et al.) relates to surface reflections at the air interfaces of the multilayer reflecting polarizer. Unless the polarizer is laminated on both sides to an existing glass component or to another existing film with clear optical adhesive, such surface reflections will reduce the transmission of light of the desired polarization in the optical system. Thus, in some cases it may be useful to add an antireflection (AR) coating to the reflecting polarizer.
Reflective polarizers are often used in visual display systems such as liquid crystal displays. These systems—now found in a wide variety of electronic devices such as mobile phones, computers, and some flat panel TVs—use a liquid crystal (LC) panel illuminated from behind with an extended area backlight. The reflective polarizer is placed over or otherwise incorporated into the backlight to transmit light of a polarization state useable by the LC panel from the backlight to the LC panel. Light of an orthogonal polarization state, which is not useable by the LC panel, is reflected back into the backlight, where it can eventually be reflected back towards the LC panel and at least partially converted to the useable polarization state, thus “recycling” light that would normally be lost, and increasing the resulting brightness and overall efficiency of the display.
A representative visual display system 20 is shown in schematic side view in FIG. 2. The system 20 includes an LC panel 22 and an illumination assembly or backlight 24 positioned to provide light to the LC panel 22. The LC panel 22 includes a layer of liquid crystal disposed between glass panel plates. The LC panel 22 is positioned between an upper absorbing polarizer 26 and a lower absorbing polarizer 28. The absorbing polarizers 26, 28 and the LC panel 22 in combination control the transmission of light from the backlight 24 through the display system 20 to the viewer. Selective activation of different pixels of the liquid crystal layer by an electronic display controller results in the light passing out of the display system 20 at the selected pixels, thus forming an image seen by the viewer.
The backlight 24 includes light sources, whether disposed in an edge-lit configuration (light source 30a) or a direct-lit configuration (light sources 30b), and distributes light from the sources over an output area that matches the viewable area of the LC panel 22. The light sources may be cold cathode fluorescent lamps (CCFLs) or light emitting diodes (LEDs), for example, and either individually or in combination they produce white light. The backlight 24 also includes a film stack generically depicted at 32, which may include various optical components such as a diffuser plate, prismatic brightness enhancement film (BEF), and the multilayer reflective polarizer discussed above. The backlight includes an enclosure whose inner bottom surface 34a and inner side surfaces 34b can be reflective to promote light recycling and enhance system efficiency. In some cases the backlight may also incorporate a solid light guide to transport light from edge-mounted light sources (light source 30a) evenly over the output area.
In any case, the backlight provides an extended light source that the LC panel 22 uses to produce an image that can be perceived by the viewer, who may be observing from on-axis (normal or near-normal) viewing directions (viewer 36a, positioned along the z-axis which is perpendicular to the multilayer reflective polarizer and to the other extended optical components of the system 20), or from off-axis or oblique viewing directions (viewer 36b).
One measure of performance of the reflective polarizer in the context of a display system such as system 20 is referred to as “gain”. The gain of a reflective polarizer or other optical film is a measure of how much brighter the display appears to the viewer with the optical film compared to the display without the optical film. More specifically, the gain of an optical film is the ratio of the luminance of the display system (or of a portion thereof, such as the backlight) with the optical film to the luminance of the display system without the optical film. Since luminance is in general a function of viewing orientation (see e.g. viewers 36a, 36b in FIG. 2), gain is also a function of viewing orientation. If gain is referred to without any indication of orientation, on-axis performance is ordinarily presumed. High gains are normally associated with reflective polarizers that have very high reflectivity for the block axis and very high transmissivity (very low reflectivity) for the pass axis, for both normally and obliquely incident light. This is because a very high block axis reflectivity maximizes the chance that a light ray of the non-useable polarization will be reflected back into the backlight so that it can be converted to the useable polarization; and a very low pass axis reflectivity maximizes the chance that a light ray of the useable polarization will pass out of the backlight towards the LC panel, with minimal loss.
Another performance measure of the reflective polarizer in the context of a full RGB color display system is the amount of color the component introduces into the system, both on-axis and off-axis, as a result of spectral non-uniformities in reflectance or transmission. Ideally, a polarizer reflects and transmits uniformly over the entire visible spectrum from about 400 to 700 nm so that it introduces no significant perceived color into the display, either on-axis or off-axis. This is most easily achieved if, again, the block axis reflectivity is as high as possible and the pass axis reflectivity is as small as possible, or more precisely, if the portion of the pass axis reflectivity due to interference effects from the microlayers is as small as possible. (The remaining portion of the pass axis reflectivity, which is due to Fresnel surface reflections at the front and back major surfaces of the polymeric reflective polarizer exposed to air, has virtually no impact on color since such air-to-polymer surface reflections are substantially spectrally uniform.) Microlayer stacks that have neither very small nor very large reflectivities are more difficult to control for color over the visible spectrum. This is because at intermediate reflectivities, even very small variations in the layer thickness profile of the stack, relative to an ideal or target thickness profile, can easily produce spectral deviations from a target flat reflection spectrum that can be readily perceived by the human eye in transmitted or reflected light.
In keeping with the above considerations, two commercially available multilayer reflective polarizer products, described in more detail below, are able to achieve good gain and low color characteristics using film designs that are different in some respects but that both have on-axis pass axis reflectivities that are very low by keeping Δny very small.